Due to the excellent thermal and electrical insulation properties of mica, it lends itself readily to use in such things as electrical insulation tape. Synthetic and natural mica can be split into thin flexible platelets which, when laid on a paper making machine overlap each other, thus forming a continuous sheet of an integrated type of mica paper. Generally, this is done by forming a slurry of the mica platelets and depositing them on a continuous belt with subsequent extraction of the water by vacuum.
Such paper is quite flimsy and easily flaked, with a tensile strength of approximately 2 to 8 pounds per inch (357.2 to 1428.8 grams per cm) of width. The only adhesive at this point is a result of the intimate contact between the smooth surfaces of the mica platelets which allows the short range Van der Waal forces to provide a cohesive bond between adjacent surface molecules. The incorporation of a polar solvent tends to disrupt the platelet orientation and in excess will return the mica to slurry form. Accordingly, further processing is necessary to provide reinforcement. Normally, this requires the impregnation of the mica paper with a resin, lamination of plies of the mica paper together, or the addition of a backing substrate to the mica paper. Note U.S. Pat. Nos. 2,562,004; 2,835,642; 4,157,414; and commonly assigned, copending U.S. patent application Ser. No. 82,250, filed Oct. 5, 1979, now U.S. Pat. No. 4,286,010, which disclose the coating and lamination of mica paper in the manufacture of electrical insulation, the disclosure of which is incorporated by reference.
Since polymer promoted adhesion is such an important factor in forming insulating mica sheets, the ability of the polymer to efficiently contact and wet the surface of the mica sheets becomes very important. The more intimate contact the liquid resin makes with the irregularities of the mica surface, the greater will be the final adhesive bond. On the other hand, poor wetting of the mica sheets tends to produce a greater stress concentration at the free surface of the adhesive where failure is most likely to be initiated. This is due to the stress concentration being much higher in the mean stress across the mica sheet. In addition, when water reaches the microscopic voids, migration proceeds through capillary action. Since water can effectively wet out the mica surface, the results are lowered surface energy and propagation of interfacial separation.
The shingle-like configuration of the mica paper decreases the percentage of platelet surface areas which come into contact with each other. Another factor which reduces contact is the adherent edge frame of the platelets themselves. Small edge pieces deviating from the plane of the surface are a product of the initial water jet delamination process of the block mica. Foreign particles such as sand, which may be carried through the wash process, also impose separation between platelets. As a result, the mica sheet substrate whose volume can consist of between 40% to 50% random void spaces and irregular plane dimensions does not readily lend itself to intimate contact by bulky polymers.
From an electrical insulation viewpoint, the presence of voids in a high-voltage system allows these voids to become sites for internal ionization and subject to intense corona discharge. This leads to an increase in the dissipation factor "tip-up" characteristics and in some cases, can even result in the generation of internal heat above normal operating temperatures. This in turn, accelerates the thermal degradation of the resin system and ultimately reduces the life of the insulation.
Attempts at improving physical properties of mica sheets have included such things as treatment of mica paper with organo titanates (note U.S. Pat. No. 2,948,640). And in fact, mica sheet-containing toaster boards treated with silicone resin and organo titanates to improve moisture resistance have been on the market for years. However, the application of this technology to mica insulation tapes has been noticeably absent from this art.
Because of the constant changes in the electrical insulation industry and the size and output of various motor systems, there exists a constant search for insulation systems which will increase operating temperatures of the various motors produced without sacrificing efficiency. A system which would overcome the above noted problems would represent a definite advancement in this art.